In a saucepan, toast the coriander and fennel seeds over moderate heat until fragrant, about 2 minutes; transfer to a mortar and let cool. Pound until coarsely ground.

Warm the 1/4 cup of oil in the same saucepan. Add the spices along with the lemon juice and let stand for 1 minute. Add the tomatoes, chopped fennel, chervil and shredded basil; season with salt and pepper.

Light a grill or preheat a grill pan. Brush the scallops with oil and season with salt and pepper. Grill the scallops over high heat, turning once, until browned and just firm, about 4 minutes. Transfer the scallops to plates and spoon the warm tomato dressing on top. Garnish with basil leaves and serve.